Well you know what you have to try now"What happens if you do not power the STR PCB but leave the cable connected to J104 ? (trying to eliminate grounding issues)"
BUT!! BREAK THROUGH : Trying that, I plugged it back in after the boot ( while in the chihiro test menu) and on exit, it went in the game and TADAM!! FFB works!!
I reproduced the following steps:
-Power off, connector J103 (+5,+12) on FFB connected, CN5 connected, power On : Result: ERROR 26
-Power off, connector J103 on FFB disconnected, CN5 connected, power on : after chihiro boot, hot plug J103, goes in, check wheel , and works!
what the !
Take J103 and disconnect a wire one by one then power on. I would make a small interface cable to avoid thrashing the original cable.
I would probably start with testing GND